In general, with an anti-lock control device for a vehicle, anti-lock control is effected by means of a microcomputer such that hold valves and decay valves are opened and closed on the basis of electrical signals representing wheel speeds detected by wheel speed sensors, thereby increasing, holding or decreasing the brake hydraulic pressure, for the purpose of securing improved steering performance and running stability, while at the same time reducing the braking distance of the vehicle.
FIG. 1 shows control state diagrams as disclosed in U.S. Pat. No. 4,741,580 which illustrate the changes in the wheel speed Vw, the wheel acceleration and deceleration Vw and the brake hydraulic pressure Pw, as well as a hold signal HS and a decay signal DS for opening and closing the hold valves and the decay valves.
In a state of the vehicle in running where no brake is operated, the brake hydraulic pressure Pw is not increased and both of the hold signal HS and the decay signal DS are in the off-state, so that the hold valve is in the open state whereas the decay valve is in the closed state. However, with a brake operation, the brake hydraulic pressure Pw increases rapidly from time point t0 (normal mode), reducing the wheel speed Vw. There is set up a reference wheel speed Vr which is lower by a predetermined amount .DELTA.V than the wheel speed Vw and follows the latter with such a speed difference. The reference wheel speed Vr is set up so that when the wheel deceleration (negative acceleration) Vw of the wheel attains a predetermined threshold value, -1G, for instance, at a time point t1, it decreases linearly in time from the time point t1 with a slope .theta. for the deceleration of -1G.
At a time point t2 when the wheel declaration Vw reaches a predetermined value -Gmax with maximum absolute value, the hold valve closes by turning on the hold signal HS to hold the brake hydraulic pressure Pw.
With the holding of the brake hydraulic pressure Pw in such a manner, the wheel speed Vw further decreases to become less than the reference wheel speed Vr beyond a time point t3. At that time point t3 the decay signal DS is turned on to open the decay valve to start reducing the brake hydraulic pressure Pw. As a result of the pressure reduction, the wheel speed Vw is shifted from decrease to increase at a time point t4 when a low peak Vl of the wheel speed Vw occurs. At the time point t4 of the low peak, the decay signal DS is turned off to close the decay valve, so that the reduction of the brake hydraulic pressure Pw is completed and the brake hydraulic pressure Pw is held at the value at that time.
Next, when the wheel speed Vw attains a high peak Vh at a time point t7, an increase in the brake hydraulic pressure takes place again. The pressure increase in the brake hydraulic pressure Pw and the decrease in the wheel speed Vw in this stage is arranged to take place gradually by a repetition of turning on and off of the hold signal HS mincingly. Starting at a time point t8 (corresponding to t3) a decompression mode is generated again.
It is to be noted that during the above operation, a time point t5 is detected at which the wheel speed Vw is recovered to a speed Vb(=Vl+0.15Y) where Vl is the wheel speed at the low peak and Y is the difference between the wheel speed Va at the time point t3 and the low peak speed Vl, so that Vb represents the wheel speed at which 15% of the speed difference Y is gained from the low peak value Vl. Also, a time point t6 is detected at which the wheel speed increases to Vc(=Vl+0.8Y) where 80% of the speed difference Y is gained from the low peak speed Vl. Further, the interval Tx of the first pressurization which starts at the time point t7 is determined by the judgment of the friction coefficient .mu. of the road surface as obtained based on the computation of the average acceleration (Vc-Vb)/.DELTA.T for the period .DELTA.T between the time points t5 and t6. In addition, the holding periods or the pressurization periods that follow are determined based on the vehicle deceleration Vw that are detected immediately before each holding or pressurization. Through a combination of augmentation, holding and reduction of the brake hydraulic pressure Pw as described in the above, it is possible to reduce the vehicle speed by controlling the wheel speed Vw without causing the locking of the wheels.
Now, as is clear from the above description, in the conventional anti-lock control method, the threshold value of deceleration at which the reference wheel speed Vr is to be changed to have a certain deceleration slope in order to increase the S/N rate in consideration of the road surface noise and the like, is selected at a value, -1 G, for example, which has a greater absolute value than that of the vehicle speed generated in the normal deceleration. Then, the reference wheel speed Vr is reduced starting with the time point t1 with the deceleration slope .theta. for -1G based on the detection of the predetermined threshold value -1G of deceleration of the wheel speed Vw, and the reduction of the brake hydraulic pressure Pw starts from the time point t3 at which the wheel speed Vw becomes equal to the reference wheel speed Vr. Because of this, if a gentle braking is performed such that the wheel speed Vw decreases with a deceleration Vw which will not attain the predetermined value of -1G, -0.7 G, for example, the reference speed Vr which merely follows the wheel speed Vw with a speed difference of .DELTA.V will never come to cross the wheel speed Vw. Then, speed reduction will continue independent of the vehicle speed without detecting the pressure reduction point for the brake hydraulic pressure Pw. As a result, there was a possibility of generating a premature locking of the wheels in case when the vehicle runs on a road surface with a small value of friction coeficient .mu..